Ferroelectric film, ferroelectric capacitor, ferroelectric memory, piezoelectric element, semiconductor element, method of manufacturing ferroelectric film, and method of manufacturing ferroelectric capacitor

ABSTRACT

A ferroelectric film is formed by an oxide that is described by a general formula AB 1-x Nb x O 3 . An A element includes at least Pb, and a B element includes at least one of Zr, Ti, V, W, Hf and Ta. The ferroelectric film includes Nb within the range of: 0.05≦x&lt;1. The ferroelectric film can be used for any of ferroelectric memories of 1T1C, 2T2C and simple matrix types.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Application No. 092,800, filed Mar.8, 2002, which application is incorporated herein by reference in itsentirety.

The disclosures of Japanese Patent Application No. 2002-309487, filed onOct. 24, 2002, Japanese Patent Application No. 2003-76129, filed on Mar.19, 2003, Japanese Patent Application No. 2003-85791, filed on Mar. 26,2003, Japanese Patent Application No. 2003-294072 filed on Aug. 18,2003, and Japanese Patent Application No. 2003-302900 filed on Aug. 27,2003 are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a ferroelectric film, a ferroelectriccapacitor, a ferroelectric memory, a piezoelectric element, asemiconductor element, a method of manufacturing a ferroelectric film,and a method of manufacturing a ferroelectric capacitor.

It has recently become popular to perform research and development intoferroelectric films of PZT or SBT or the like, as well as devices suchas ferroelectric capacitors and ferroelectric memory devices that usesuch films. The configurations of ferroelectric memory devices arecategorized into 1T type, 1T1C type, 2T2C type, and simple matrix type.Of these, the structure of the 1T type leads to the generation ofinternal electrical fields which shorten the retention (datapreservation) to one month, so it is thought to be impossible to providea guarantee of ten years, which is generally requested ofsemiconductors. The 1T1C type and the 2T2C type have mostly the sameconfiguration as DRAM and have selection transistors, so DRAMfabrication techniques can be used therefor. Since the 1T1C type and the2T2C type implement write speeds similar to those of SRAM, they arecurrently being used in small-capacitance capacitance products of 256Kbit or less.

The ferroelectric materials used up until now have mainly been Pb(Zr,Ti)O₃ (PZT). With PZT, the ratio of Zr to Ti is 52/48 or 40/60 and thecomposition that is used is a region that is a mixture of trigonal andtetragonal crystals, or the vicinity thereof. With PZT, materials thathave also been doped with elements such as Lz, Sr, or Ca are used. Theseregions are used because they guarantee the reliability that is mostnecessary for a memory element. A tetragonal region that is rich in Tihas a favorable hysteresis shape, but Schottky defects caused by theionic crystal structure occur therein. For that reason, failures occurin the leakage current characteristic or imprint characteristic(measures of hysteresis distortion), making it difficult to ensurereliability.

A simple matrix type of memory cell, on the other hand, has a cell sizesmaller than those of the 1T1C and 2T2C types, and it is also possibleto stack capacitors, so it is promising for high integration,inexpensive applications.

Details of a conventional simple matrix type of ferroelectric memorydevice are given in Japanese Patent Laid-Open No. 9-116107. Thispublication discloses a drive method by which a voltage that is ⅓ of thewrite voltage is applied to non-selected memory cells when data iswritten to the memory cells.

However, details concerning the hysteresis loop of the ferroelectriccapacitor, which is necessary for operation, are not specificallydisclosed therein. Good squareness of hysteresis loop is essential forobtaining a simple matrix type of ferroelectric memory device that canoperate in practice. Ti-rich tetragonal PZT can be considered as acandidate for the ferroelectric material that can be applied thereto,but the guaranteeing of reliability is the most important technicalconcern therewith, in a similar manner to the 1T1C and 2T2C types offerroelectric memory.

PZT tetragonal crystals exhibit a hysteresis characteristic that has thesquareness suitable for memory applications, but they lack reliabilityand cannot be used in practice. The reasons for this are discussedbelow.

First of all, a PZT tetragonal thin film tends to have a high leakagecurrent density after crystallization, which increases as the ratio ofTi contained therein increases. In addition, static imprint testing inwhich data is written once in either the positive or negative directionand the memory device is heated and held at 100° C. has shown that mostof the written data disappears after 24 hours. These problems areintrinsic to the ionic crystals of PZT and to the Pb and Ti that areconstituent elements of PZT, and create the greatest technical problemrelating to PZT tetragonal thin film in which large proportions of theconstituent elements are Pb and Ti. This technical problems is greatbecause PZT Perovskite is ionic crystals, and is intrinsic to PZT.

A list of the main energies involved in the bonds between theconstituent elements of PZT is shown in FIG. 44. It is known that PZTincludes many oxygen vacancies after crystallization. In other words, itcan be expected from FIG. 44 that Pb—O bonds have the smallest bondenergy among the constituent elements of PZT and will simply breakduring baking or polarization inversions. In other words, if Pb escapes,O will also escape for reasons of charge neutrality.

During sustained heating such as imprint testing, the constituentelements of PZT vibrate and collide repeatedly, and the Ti that is thelightest constituent element of PZT can easily be knocked out by thesevibrational collisions during high-temperature retention. Therefore, ifTi escapes, O will also escape for reasons of charge neutrality. Sincethe maximum valence of +2 for Pb and +4 for Ti contribute towardsbonding, there is no way to maintain charge neutrality other thanallowing O to escape. In other words, two negative O ions escape readilyfor every positive ion of Pb or Ti, so that Schottky defects easilyform.

The description now turns to the mechanism of the generation of leakagecurrents due to oxygen lack in PZT crystals. FIGS. 45A to 45C illustratethe generation of leakage currents in oxide crystals having aBrownmillerite type of crystal structure described by the generalformula ABO_(2.5). As shown in FIG. 45A, the Brownmillerite type ofcrystal structure is a crystal structure having an oxygen insufficiencyin comparison with the Perovskite type of crystal structure of PZTcrystals having the general formula ABO₃. As shown in FIG. 45B, sinceoxygen ions appear in the vicinity of positive ions in theBrownmillerite type of crystal structure, positive ion defects make itdifficult for excessive leakage current to increase. However, oxygenions link the entire PZT crystal in series as shown in FIG. 45C, andleakage currents increase accordingly in the case of a Brownmilleritetype of crystal structure, in which the oxygen vacancy is larger thanthe above description.

In addition to the above-described generation of leakage currents,insufficiencies of Pb and Ti and the concomitant insufficiency of O,which are lattice defects, cause spatial charge polarization such asthat shown in FIG. 46. When that happens, reverse electrical fields dueto lattice defects are created by the electrical fields of ferroelectricpolarization can occur, causing a state in which the bias potential isimpeded in the PZT crystals, and hysteresis shift or collapse can occuras a result. Furthermore, these phenomena are likely to occur quicker asthe temperature increases.

The above problems are intrinsic to PZT and it is considered difficultto analyze these problems in pure PZT, so that up until now it has notbeen possible to implement suitable characteristics for a memory elementmade by using tetragonal PZT.

In ferroelectric memory, one factor that determines the characteristicsof the device is the crystallization state of the ferroelectric filmincluded within the ferroelectric capacitor. The process ofmanufacturing ferroelectric memory has processes for forming aninterlayer dielectric and a protective film, and processes that generatelarge quantities of hydrogen are used. Since the ferroelectric film atthis point is mainly formed of oxides, the oxides are reduced by thegenerated hydrogen during the fabrication process, which has anundesirable effect on the characteristics of the ferroelectriccapacitor.

For that reason, a resistance to reduction is secured for theferroelectric capacitor in the conventional ferroelectric memory bycovering the capacitor with a barrier film such as an aluminum oxidelayer or an aluminum nitride layer, to prevent deterioration of thecharacteristics of the ferroelectric capacitor. However, such a barrierfilm necessitates the use of extra real estate during the integration ofthe ferroelectric memory, making it desirable to find a method thatenables the manufacture of ferroelectric memory by a simpler process,from the productivity point of view as well.

BRIEF SUMMARY OF THE INVENTION

The present invention may provide a 1T1C, 2T2C, or simple matrix type offerroelectric memory including a ferroelectric capacitor having ahysteresis characteristic that can be used in any of a 1T1C, 2T2C, orsimple matrix type of ferroelectric memory. The present invention mayalso provide a ferroelectric film that is suitable for theabove-described ferroelectric memory, together with a method ofmanufacturing the same. The present invention may further provide apiezoelectric element and semiconductor element in which theabove-described ferroelectric film is used. The present invention maystill further provide a ferroelectric capacitor, a method of manufacturethereof, and a ferroelectric memory in which the ferroelectric capacitoris used, wherein satisfactory characteristics are maintained by a simpleprocess that does not necessitate a barrier film.

A ferroelectric film according to one aspect of the present invention isdescribed by a general formula AB_(1-x)Nb_(x)O₃, an A element includesat least Pb, a B element includes at least one of Zr, Ti, V, W, Hf andTa, and Nb is included within the range of: 0.05≦x<1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic section through a ferroelectric capacitor;

FIG. 2 is a flowchart of the formation of a PZTN film by a spin-coatingmethod

FIG. 3 is a hysteresis curve of polarization (P) versus voltage (V) ofthe ferroelectric capacitor;

FIGS. 4A to 4C show the surface morphologies of PZTN films in accordancewith a first embodiment;

FIGS. 5A to 5C show the crystallinities of PZTN films in accordance withthe first embodiment;

FIGS. 6A to 6C show the relationship between film thickness and surfacemorphology of PZTN films in accordance with the first embodiment;

FIGS. 7A to 7C show the relationship between film thickness andcrystallinity of PZTN films in accordance with the first embodiment;

FIGS. 8A to 8C show the hysteresis characteristics for film thicknessesof PZTN films in accordance with the first embodiment;

FIGS. 9A to 9C show the hysteresis characteristics for film thicknessesof PZTN films in accordance with the first embodiment;

FIGS. 10A and 10B show the leakage current characteristics of PZTN filmsin accordance with the first embodiment;

FIG. 11A shows the fatigue characteristic of a PZTN film in accordancewith the first embodiment, and FIG. 11 shows the static imprintcharacteristic of a PZTN film in accordance with the first embodiment;

FIG. 12 shows the configuration of a ferroelectric capacitor inaccordance with the first embodiment in which a SiO₂ protective film isformed by ozone TEOS;

FIG. 13 shows the hysteresis characteristic of the ferroelectriccapacitor in accordance with the first embodiment in which a SiO₂protective film is formed by ozone TEOS;

FIGS. 14 shows the leakage current characteristics of a conventionalPZTN film;

FIG. 15 shows the fatigue characteristic of a ferroelectric capacitorusing a conventional PZTN film;

FIG. 16 shows the static imprint characteristic of a ferroelectriccapacitor in accordance with the first embodiment, which uses aconventional PZT film;

FIGS. 17A and 17B show the hysteresis characteristics of PZTN films inaccordance with a second embodiment.

FIGS. 18A and 18B show the hysteresis characteristics of PZTN films inaccordance with a second embodiment.

FIGS. 19A and 19B show the hysteresis characteristics of PZTN films inaccordance with a second embodiment.

FIG. 20 shows X-ray diffraction patterns of PZTN films in accordancewith the second embodiment;

FIG. 21 shows the relationship between Pb insufficiency and Nbcompositional ratio in a PZTN crystal in accordance with the secondembodiment;

FIG. 22 is illustrative of the WO₃ crystal structure that is aPerovskite crystal;

FIGS. 23A to 23C are schematic sections illustrating the process ofmanufacturing a PZTN film in accordance with a third embodiment;

FIGS. 24A and 24B are illustrative of changes in lattice constant in aPZTN film in accordance with the third embodiment;

FIG. 25 is illustrative of changes in lattice mismatch ratio betweenPZTN films and Pt metal films in accordance with the third embodiment;

FIG. 26 is a flowchart of the formation of a conventional PZT film by aspin-coating method, as a reference example;

FIGS. 27A to 27E show the surface morphologies of PZTN films, as areference example;

FIGS. 28A to 28E show the crystallinities of PZTN films, as a referenceexample;

FIGS. 29A and 29B show the hysteresis loops of tetragonal PZT films, asreference examples;

FIG. 30 shows the hysteresis loop of a conventional tetragonal PZT film,as a reference example;

FIGS. 31A and 31B show the results of degassing analysis on tetragonalPZT films as reference examples;

FIGS. 32A to 32C show a process of manufacturing a ferroelectriccapacitor;

FIGS. 33A and 33B show the hysteresis characteristics of ferroelectriccapacitors;

FIG. 34 shows the electrical characteristics of ferroelectric capacitors

FIG. 35A is a schematic plan view of a simple matrix type offerroelectric memory device and FIG. 35B is a schematic section throughthe simple matrix type of ferroelectric memory device;

FIG. 36 is a section through an example of a ferroelectric memory devicein which a memory cell array and a peripheral circuit are integratedtogether on the same substrate;

FIG. 37A is a schematic section through a 1T1C type of ferroelectricmemory and FIG. 37B is an equivalent circuit schematically showing the1T1C type of ferroelectric memory;

FIGS. 38A to 38C show the process of manufacturing ferroelectric memory;

FIG. 39 is an exploded perspective view of a recording head;

FIG. 40A is a plan view of the recording head and FIG. 40B is a sectionthrough the recording head;

FIG. 41 is a schematic section through the layer structure of apiezoelectric element;

FIG. 42 is a schematic view of an example of an inkjet-type recordingdevice;

FIG. 43A shows the hysteresis characteristic of a ferroelectric film inwhich Ta has been added to PZT and FIG. 43B shows the hysteresischaracteristic of a ferroelectric film in which W has been added to PZT;

FIG. 44 lists characteristics relating to bonds of constituent elementsof PZT-family ferroelectric materials;

FIGS. 45A to 45C are illustrative of Schottky defects in theBrownmillerite crystal structure; and

FIG. 46 is illustrative of ferroelectric spatial charge polarization.

DETAILED DESCRIPTION OF THE EMBODIMENT

1) A ferroelectric film according to an embodiment of the presentinvention is described by a general formula AB_(1-x)Nb_(x)O₃, an Aelement includes at least Pb, a B element includes at least one of Zr,Ti, V, W, Hf and Ta, and Nb is included within the range of: 0.05≦x<1.

The A element may include Pb_(1-y)Ln_(y) (0<y≦0.2). Ln includes at leastone of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

2) A ferroelectric film according to an embodiment of the presentinvention is described by a general formula(Pb_(1-y)A_(y))(B_(1-x)Nb_(x))O₃, and an A element includes at least oneof La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu, a Belement includes at least one of Zr, Ti, V, W, Hf and Ta, and Nb isincluded within the range of: 0.05≦x<1 (more desirably 0.1≦x≦0.3).

3) With a PZT-family ferroelectric film according to an embodiment ofthe present invention, a Ti composition is greater than a Zrcomposition, and at least 2.5 mol % and not more than 40 mol % (moredesirably at least 10 mol % and not more than 30 mol %) of the Ticomposition is substituted by Nb. This PZT-family ferroelectric film mayhave a crystal structure of at least one of tetragonal and rhombohedralsystems. This PZT-family ferroelectric film may include Si, or Si and Geof at least 0.5 mol % (more desirably at least 0.5 mol % and less than 5mol %).

This PZT-family ferroelectric film may be formed by using a sol-gelsolution.

4) A PZT-family ferroelectric according to an embodiment of the presentinvention is described by a general formula ABO₃, Pb is included as aconstituent element in an A site and at least Zr and Ti are included asconstituent elements in a B site. Amount of Pb vacancy in the A site isequal to or less than 20 mol % of the stoichiometric composition of theABO₃. This PZT-family ferroelectric film may include Nb in the B sitewith a compositional ratio equivalent to twice the Pb vacancy in the Asite. With this PZT-family ferroelectric film, a Ti composition may behigher than a Zr composition in the B site, and also the ferroelectricmay have a crystal structure of rhombohedral system. This PZT-familyferroelectric film may be formed by using a sol-gel solution.

5) With a method of manufacturing the above ferroelectric film accordingto an embodiment of the present invention, a mixture of at least asol-gel solution for PbZrO₃, a sol-gel solution for PbTiO₃, and asol-gel solution for PbNbO₃ is used as the sol-gel solution for formingthe ferroelectric film.

With this method of manufacturing a ferroelectric film, a sol-gelsolution for forming PbSiO₃ may be further mixed into the mixture to beused as the sol-gel solution for forming the ferroelectric film.

6) With a method of manufacturing the above ferroelectric film accordingto an embodiment of the present invention, when the stoichiometriccomposition of Pb that is a constituent element of the A site is assumedto be 1, the ferroelectric film is formed by using a sol-gel solution inwhich Pb is included within the range of 0.9 to 1.2.

7) With this method of manufacturing the ferroelectric film, thePZT-family ferroelectric film may be formed on a metal film formed of aplatinum-group metal.

8) With this method of manufacturing the ferroelectric film, theplatinum-group metal may be at least one of Pt and Ir.

9) The ferroelectric memory in accordance with an embodiment of thepresent invention includes a first electrode leading up from the sourceor drain electrode of a CMOS transistor that has been formed on an Siwafer previously, a ferroelectric film formed on the first electrode,and a second electrode formed on the ferroelectric film. A capacitorformed of the first electrode, the ferroelectric film and the secondelectrode is a ferroelectric memory that performs a selection operationby the CMOS transistor that was formed on an Si wafer in advance. Theferroelectric film is formed from tetragonal PZT having a Ti ratio of atleast 50%, at least 5 mol % but not more than 40 mol % of the Ticomposition is substituted by Nb, and Si and Ge of at least 1 mol % isincluded therein.

10) The ferroelectric memory in accordance with an embodiment of thepresent invention is a ferroelectric memory including a previouslyformed first electrode, a second electrode arranged in a directionintersecting the first electrode, and a ferroelectric film disposed inat least an intersecting region between the first electrode and thesecond electrode. Capacitors form of the first electrode, theferroelectric film, and the second electrode are disposed in a matrix.The ferroelectric film is formed from tetragonal PZT having a Ti ratioof at least 50%, at least 5 mol % but not more than 40 mol % of the Ticomposition is substituted by Nb, and Si and Ge of at least 1 mol % isincluded therein.

11) A method of manufacturing a ferroelectric memory in accordance withan embodiment of the present invention includes crystallizing a sol-gelsolution for forming PbZrO₃ that is a first raw material solution, asol-gel solution for forming PbTiO₃ that is a second raw materialsolution, a sol-gel solution for forming PbNbO₃ that is a third rawmaterial solution, and sol-gel solution for forming PbSiO₃ that is afourth raw material solution, after the first to fourth solutions havebeen coated. The first, second, and third raw material solutions areliquids of raw materials for forming the ferroelectric layer and thefourth raw material solution is a liquid of a raw material for formingan ordinary paraelectric layer having a catalytic effect that isessential for forming the ferroelectric layer from the first, second,and third raw material solutions.

12) A method of manufacturing a ferroelectric capacitor according to anembodiment of the present invention includes:

forming a lower electrode on a given substrate;

forming a ferroelectric film on the lower electrode, the ferroelectricfilm being formed of a PZTN complex oxide including Pb, Zr, Ti and Nb asconstituent elements;

forming an upper electrode on the ferroelectric film;

forming a protective film so as to cover the lower electrode,ferroelectric film, and upper electrode; and

performing thermal processing for crystallizing the PZTN complex oxide,at least after forming the protective film.

This embodiment uses a PZTN complex oxide that includes Pb, Zr, Ti andNb as constituent elements as the material of the ferroelectric film,and this PZTN complex oxide is crystallized after the formation of aprotective film. Thus, even if the ferroelectric film should becomedamaged by hydrogen generated during the processing in the formation ofthe protective film, the thermal processing for crystallization isperformed subsequently so that the PZTN complex oxide is crystallizedwhile any such damage is repaired. It is therefore possible to omit theprocess of forming a barrier layer to protect the ferroelectric filmfrom reductive reactions, which is necessary in the prior art, thusenabling an increase in productivity and a reduction in productioncosts.

13) With this method of manufacturing a ferroelectric capacitor,preliminary thermal processing may be performed on the ferroelectricfilm in an oxidizing atmosphere during the formation of theferroelectric film, to put the PZTN complex oxide into an amorphousstate until thermal processing for crystallizing the PZTN complex oxideis performed.

This feature enables an amorphous state until the ferroelectric film hasbeen crystallized. This makes it possible to prevent deterioration ofthe crystal quality due to grain boundary diffusion by keeping theferroelectric film in the amorphous state until the protective film isformed. Since the ferroelectric film in this amorphous state issubjected to the preliminary thermal processing in an oxidizingatmosphere, oxygen can enter the film. For that reason, it is possibleto crystallize the PZTN complex oxide during the thermal processing forcrystallization without depending on the gases included within theatmosphere therefor.

14) With this method of manufacturing a ferroelectric capacitor, theprotective film may be a silicon dioxide film and is formed by usingtrimethylsilane.

Since this feature makes it possible to form the protective film ofsilicon dioxide film by using trimethylsilane (TMS) that does notgenerate much hydrogen during the processing, in comparison with thetetraethyl orthosilicate (TEOS) that is generally used in the formationof silicon dioxide films, it is possible to reduce damage due toreductive reactions in the ferroelectric film.

15) With this method of manufacturing a ferroelectric capacitor, thethermal processing for crystallizing the PZTN complex oxide may beperformed in a non-oxidizing atmosphere.

Since this feature makes it possible to perform the thermal processingfor crystallization in a non-oxidizing atmosphere, it makes it possibleto prevent oxidation damage due to high-temperature thermal processingon peripheral components outside the capacitor (such as metal wiring),even if such peripheral components are included in the device beingprocessed.

16) A ferroelectric capacitor according to an embodiment of the presentinvention is manufactured by using the above manufacture method of theferroelectric capacitor.

17) The above ferroelectric film and ferroelectric capacitor can beapplied to a ferroelectric memory, piezoelectric element, andsemiconductor element using the same. Preferred embodiments of thepresent invention are described below in detail with reference to theaccompanying figures.

1. Ferroelectric Film, Ferroelectric Capacitor, and Method ofManufacture Thereof

A schematic section through a ferroelectric capacitor 100 that uses aferroelectric film 101 in accordance with an embodiment of the presentinvention is shown in FIG. 1.

As shown in FIG. 1, the ferroelectric capacitor 100 is formed of theferroelectric film 101, a first electrode 102, and a second electrode103.

The first electrode 102 and the second electrode 103 are either formedof a precious metal such as Pt, Ir, or Ru alone, or a compound materialin which that precious metal is the main part. Since the diffusion offerroelectric elements into the lower electrode 102 or the upperelectrode 103 would cause variations in the composition of the interfacebetween that electrode and the ferroelectric film 101, which wouldadversely affect the squareness of the hysteresis loop, a compactstructure that does not permit the diffusion of ferroelectric elementsinto the lower electrode 102 or the upper electrode 103 is desired.Among methods of increasing the compactness of the lower electrode 102and the upper electrode 103 is a method of forming the films bysputtering by a gas having a large mass, or a method of dispersing anoxide of a substance such as Y or L into a precious metal electrode.

The ferroelectric film 101 is formed by using a PZT-family ferroelectricformed of an oxide including Pb, Zr and Ti as constituent elements. Thisembodiment is particularly characterized in the use of Pb(Zr, Ti, Nb)O₃(PZTN) obtained by doping Nb into Ti sites of this ferroelectric film101.

Nb is substantially the same size as Ti (the ionic radii thereof areclose and the atomic radii are the same) but is twice the weightthereof, so the atoms thereof are unlikely to escape from the latticeeven if there are collisions between atoms due to lattice vibration. Thevalence of Nb is stable at +5, so that even if the Pb escapes, theatomic weight after the Pb has escaped can be compensated for by theNb⁵⁺. During the crystallization, even if Pb escape occurs, it issimpler for the small-sized Nb to enter than the large-sized O toescape.

Since there are also some Nb atoms of valence +4, it is possible thatthe substitution of Ti⁴⁺ will be performed sufficiently. In addition, itis thought that the covalence of Nb is extremely strong in practice,making it difficult for Pb to escape (refer to H. Miyazawa, E. Natori,S. Miyashita; Jpn. J. Appl. Phys. 39 (2000) 5679).

Up to now, the Nb doping into PZT has been mainly performed into Zr-richtrigonal crystal regions and is extremely small, on the order of 0.2 to0.025 mol % (refer to J. Am. Ceram. Soc, 84 (2001) 902 and Phys. Rev.Let, 83 (1999) 1347). The reason why it has not been possible to dopelarge quantities of Nb in this manner is considered to be because theaddition of 10 mol % of Nb, for example, would require an increase incrystallization temperature to at least 800° C.

In such a case, it is preferable to further add PbSiO₃3 silicate in theproportion of 1 to 5 mol %, for example, during the formation of theferroelectric film 101. This makes it possible to reduce thecrystallization energy of the PZTN. In other words, if PZTN is used asthe material of the ferroelectric film 101, the addition of PbSiO₃silicate makes it possible to design a reduction in the crystallizationtemperature of the PZTN.

The description now turns to an example of a film formation method forthe PZTN ferroelectric film 101 employed in the ferroelectric capacitor100 of this embodiment.

The ferroelectric film 101 can be obtained by preparing mixed solutionsformed of first to third raw material solutions including at least oneof Pb, Zr, Ti and Nb, then subjecting the oxides included within thesemixed liquids to thermal processing or the like, to cause them tocrystallize.

An example of the first raw material solution could be a solution inwhich a condensation polymer for forming PbZrO₃ Perovskite crystals byPb and Zr, from among constituent metal elements for the PZTNferroelectric phase, is dissolved in a non-aqueous state in a solventsuch as n-butanol.

An example of the second raw material solution could be a solution inwhich a condensation polymer for forming PbTiO₃ Perovskite crystals byPb and Ti, from among constituent metal elements for the PZTNferroelectric phase, is dissolved in a non-aqueous state in a solventsuch as n-butanol.

An example of the third raw material solution could be a solution inwhich a condensation polymer for forming PbNbO₃ Perovskite crystals byPb and Nb, from among constituent metal elements for the PZTNferroelectric phase, is dissolved in a non-aqueous state in a solventsuch as n-butanol.

When the ferroelectric film 101 is formed fromPbZr_(0.2)Ti_(0.8)Nb_(0.2)O₃ (PZTN) by using the above first, second,and third raw material solutions, by way of example, the ratios of (thefirst raw material solution):(the second raw material solution):(thethird raw material solution) could be 2:6:2. However, any attempt to usethese mixed solutions for crystallization as they are would necessitatea high crystallization temperature for the manufacture of the PZTNferroelectric film 101. In other words, since the mixing in of Nb willcause the crystallization temperature to rise abruptly, makingcrystallization impossible within the temperature range that enables thecreation of the component at not more than 700° C., a substitution forTi by Nb has not been used more than 5 mol % in the conventional art andit has been used only as an additive. In addition, there have beenabsolutely no examples of PZT tetragonal crystals in which there is moreTi than Zr. This is discussed in the previously cited J. Am. Ceram. Soc,84 (2001) 902 and Phys. Rev. Let, 83 (1999) 1347.

This embodiment makes it possible to solve the above-described technicalproblems by further adding to the above-described mixed solution atleast 1 mol % but less than 5 mol % of a fourth raw material solution inwhich a condensation polymer for forming PbSiO₃ crystals is dissolved ina non-aqueous state in a solvent such as n-butanol.

In other words, the use of the above-described mixture of the first,second, third, and fourth solutions makes it possible to move thecrystallization temperature of the PZTN to a practicable temperaturerange of not more than 700° C.

More specifically, the ferroelectric film 101 is formed in accordancewith the flowchart of FIG. 2. The ferroelectric film 101 is formed byrepeating a mixed solution painting process (step ST11) then a series ofan alcohol removal process, a dry thermal process, and an absorbentthermal process (steps ST12 and ST13) a desired number of times,followed by baking by crystallization annealing (step ST14).

Examples of the conditions for these processes are given below.

First of all, the film for the lower electrode is formed to cover aprecious metal for the electrode, such as Pt, on a Si substrate (stepST10). A mixed liquid is then painted thereon by a method such asspin-coating (step ST11). More specifically, the mixed solution isdropped onto the Pt-covered substrate. After spinning at approximately500 rpm with the objective of spreading the dropped solution over theentire surface of the substrate, the angular velocity is dropped to notmore than 50 rpm for about 10 seconds. The dry thermal processing isdone at 150° C. to 180° C. (step ST13). The dry thermal processing isdone by using a hot-plate or the like in the atmosphere. Similarly, theabsorbent thermal processing is done in the atmosphere on the hot-plate,which is held at 300° C. to 350° C. (step ST13). The baking forcrystallization is done by using rapid thermal annealing (RTA) or thelike in an oxygen atmosphere (step ST14).

The film thickness after baking can be on the order of 100 to 200 nm.After the upper electrode has been formed by sputtering or the like(step ST15), post-annealing is done with the objective of forming aninterface between the second electrode and the ferroelectric thin filmand improving the crystallinity of the ferroelectric thin film, usingRTA or the like in an oxygen atmosphere in a similar manner to thebaking (step ST16), to achieve the ferroelectric capacitor 100.

The effects of the use of the PZTN ferroelectric film 101 in theferroelectric capacitor 100 on the hysteresis characteristic arediscussed below.

A hysteresis curve of electric polarization (P) versus voltage (V) ofthe ferroelectric capacitor 100 is shown schematically in FIG. 3. Firstof all, when a voltage +Vs is applied, the polarization magnitude isP(+Vs), then when the voltage becomes 0 the polarization magnitudebecomes Pr. When the voltage changes to −1/3 Vs, the polarizationmagnitude is P(−1/3 Vs). When the voltage then becomes −Vs, thepolarization magnitude becomes (−Vs), and when the voltage is again 0,the polarization magnitude becomes −Pr. When the voltage becomes +1/3Vs, the polarization magnitude becomes P(+1/3 Vs), and when the voltageis again +Vs, the polarization magnitude returns to P(+Vs).

The ferroelectric capacitor 100 also has the features described below,with respect to the hysteresis characteristic. If a voltage Vs isapplied and the polarization magnitude has gone to P(+Vs) then theapplied voltage goes to 0, the hysteresis loop follows the pathindicated by the arrow A in FIG. 3 and the polarization magnitude holdsthe stable value PO(0). If a voltage −Vs is applied and the polarizationmagnitude has gone to P(−Vs) then the applied voltage goes to 0, thehysteresis loop follows the path indicated by the arrow B in FIG. 3 andthe polarization magnitude holds the stable value PO(1). Thoroughutilization of this difference between the polarization magnitude PO(0)and the polarization magnitude PO(1) makes it possible to operate asimple matrix type of ferroelectric memory device by the drive methoddisclosed in Japanese Patent Laid-Open No. 9-116107.

The ferroelectric capacitor 100 of this embodiment enables a reductionin the crystallization temperature, an improvement in the squareness ofhysteresis loop, and an improvement in Pr. The improvement in squarenessof hysteresis loop achieved by the ferroelectric capacitor 100 has theobvious effect of stabilizing major disturbances in the driving of thesimple matrix type of ferroelectric memory device. In a simple matrixtype of ferroelectric memory device, a voltage of ±1/3 Vs is appliedeven to cells that are not being written to or read, so it is necessaryto have a stable disturbance characteristic to ensure that thepolarization does not change with these voltage changes. The presentinventors have confirmed for a general PZT that a deterioration ofapproximately 80% of the polarization magnitude is seen when a 1/3 Vspulse is applied 108 times in the direction opposite to the polarizationfrom a stable polarization state, but the deterioration in theferroelectric capacitor 100 of this embodiment is not more than 10%. Theuse of the ferroelectric capacitor 100 of this embodiment in aferroelectric memory device therefore makes it possible to realize asimple matrix type of memory.

A detailed description of these embodiments is given below.

FIRST EMBODIMENT

This embodiment compares the PZTN of the present invention and the PZTof the conventional art. The entire film formation flow shown in FIG. 2was used.

Ratios of Pb:Zr:Ti:Nb=1:0.2:0.6:0.2, 1:0.2:0.7:0.1, and 1:0.3:0.65:0.5were used. In other words, the total quantity of added Nb is 5 to 20 mol%. In this case, 0 to 1% of PbSiO₃ is added.

The surface morphologies of the films in this case are shown in FIGS. 4Ato 4C. When the crystallinity of these films were measured by an X-raydiffraction method, the results were as shown in FIGS. 5A to 5C. Withthe 0% (none) case shown in FIG. 5A, only ordinary paraelectricpyrochlore is obtained, even when the crystallization temperature risesto 800° C. With the 0.5% case shown in FIG. 5B, PZT and the pyrochloreare mixed. With the 1% case shown in FIG. 5C, a single orientated filmof PZT (111) is obtained. The crystallinity thereof is also good, of aquality that can not be achieved up to now.

Next, the crystallinity of comparative examples of PZTN thin films with1% of PbSiO₃ added thereto, for different film thicknesses of 120 to 200nm, are shown in FIGS. 6A to 6C and FIGS. 7A to 7C. Note that FIGS. 6Ato 6C are electron microphotographs of the surface morphologies for filmthicknesses 120 nm to 200 nm and FIGS. 7A to 7C shown the results ofmeasurements done by an X-ray diffraction method to demonstrate thecrystallinity of PZTN thin films of film thicknesses 120 nm to 200 nm.As shown in FIGS. 8A to 8C and FIGS. 9A to 9C, hysteresischaracteristics with good squareness were obtained over the entire rangeof film thickness of 120 nm to 200 nm. Note that FIGS. 9A to 9C areenlargements of the hysteresis curves of FIGS. 8A to 8C. These resultsconfirmed that the hysteresis curves clearly opened up and also reachedsaturation at low voltages of less than or equal to 2 V, in the ZPTNthin films of these examples.

The leakage characteristics were also extremely good at 5×10⁻⁸ to 7×10⁻⁹A/cm² when 2 V (saturation) was applied thereto, regardless of the filmcomposition and film thickness, as shown in FIGS. 10A and 10B.

The results of measurements of fatigue characteristics and staticimprinting of PbZr_(0.2)Ti_(0.8)Nb_(0.2) thin films were also good, asshown in FIGS. 11A and 11B. In particular, the fatigue characteristic ofFIG. 11A is extremely good, regardless of whether Pt was used in theupper and lower electrodes.

Tests were also performed on an SiO₂ film 605 formed by ozone TEOS on aferroelectric capacitor 600 in which a lower electrode 602, a PZTNferroelectric film 603 of this embodiment, and an upper electrode 604are formed on a substrate 601, as shown in FIG. 12. It is known in theart that, if an SiO₂ film is formed by ozone TEOS on PZT, the hydrogenemitted by the TEOS passes through the upper Pt and reduces, and the PZTcrystal is so destroyed that the hysteresis phenomenon does not occur.

With the PZTN ferroelectric film 603 of this embodiment, however,favorable hysteresis is maintained with substantially no deterioration,as shown in FIG. 13. In other words, it is clear that the PZTNferroelectric film 603 of this embodiment also has a strong resistanceto reduction. If the proportion of Nb in the tetragonal PZTNferroelectric film 603 of the present invention does not exceed 40 mol%, favorable hysteresis is obtained in proportion to the quantity of Nbadded.

Evaluation with a conventional PZT ferroelectric film was done forcomparison, the conventional PZT samples had Pb:Zr:Ti ratios of1:0.2:0.8, 1:0.3:0.7, and 1:0.6:0.4. The leakage characteristics thereofare such that the leakage characteristics deteriorate with increasing T1content, as shown in FIG. 14, so that it is clear that when Ti is 80%and 2 V was applied, the characteristic was 10⁻⁵ A/cm², making itunsuitable for memory applications. Similarly, the fatiguecharacteristic deteriorated with increasing Ti content, as shown in FIG.15. After imprinting, it was clear that most of the data could not beread, as shown in FIG. 16.

As is clear from the above description, the PZTN ferroelectric film inaccordance with this embodiment has simply solved the problem of theincrease in leakage current together with the deterioration in theimprint characteristic, which are thought to be intrinsic to PZT in theconventional art, making it possible to use tetragonal PZT in memoryapplications without concern of memory type or configuration. For thesame reason, this material can also be used in piezoelectric componentapplications in which tetragonal PZT could not be used before.

SECOND EMBODIMENT

This embodiment is a comparison of the ferroelectric characteristicsobtained when the amount of Nb added to the PZTN ferroelectric film wasvaried to 0, 5, 10, 20, 30, 40 mol %. 5 mol % of PbSiO₃ was added to allthe testpieces. In addition, methyl succinate was added to the sol-gelsolutions for forming the ferroelectric films, includes of raw materialsfor film formation, to adjust the pH to 6. The entire film formationflow shown in FIG. 2 was used therefor.

Measured hysteresis characteristics of PZTN ferroelectric films inaccordance with this embodiment are shown in FIGS. 17 to 19.

FIG. 17A shows that when the quantity of added Nb is 0, leaky hysteresisis obtained, whereas FIG. 17B shows that when the quantity of added Nbis 5 mol %, a good hysteresis characteristic with a high level ofinsulation is obtained.

FIG. 18A shows that substantially no change is seen in the ferroelectriccharacteristic until the quantity of added Nb reaches 10 mol %. Evenwhen the quantity of added Nb is 0, it is leaky by no change is seen inthe ferroelectric characteristic. FIG. 18B shown that when the quantityof added Nb is 20 mol %, a hysteresis characteristic with an extremelygood squareness is obtained.

However, it has been confirmed that if the quantity of Nb added exceeds20 mol %, the hysteresis characteristic changes greatly and tends todeteriorate, as shown in FIGS. 19A and 19B.

Comparisons of X-ray diffraction patterns are shown in FIG. 20. When thequantity of added Nb is 5 mol % (Zr/Ti/Nb=20/75/5), the (111) peakposition does not change from that of a PZT film of the conventional artin which no Nb is added, but the (111) peak does shift towards thelow-angle side in accordance with the increases in the quantity of addedNb to 20 mol % (Zr/Ti/Nb=20/60/20) and 40 mol % (Zr/Ti/Nb=20/40/40). Inother words, it is clear that the actual crystal is trigonal, regardlessof whether there are Ti-rich tetragonal regions in the PZT composition.It is clear that the ferroelectric characteristics change as the crystalcomposition changes.

In addition, when the quantity of added Nb reaches 45 mol %, asufficient hysteresis loop could not be obtained and it was not possibleto confirm the ferroelectric characteristics (not shown in the figures).

It has already been stated that the PZTN of the present invention has anextremely high level of insulation, but FIG. 21 shows this from theviewpoint of obtaining conditions that ensure that the PZTN is adielectric.

In other words, the PZTN of the present invention has an extremely highlevel of insulation and this effect can be achieved by ensuring that Nbis added to Ti sites in compositional ratio equivalent to twice aninsufficiency of Pb.

With PZTN, therefore, the addition of Nb enables active control of Bbinsufficiency, and also control over the crystal configuration.

This shows that the PZTN of this embodiment would be extremely usefulwhen applied to piezoelectric element. In general, when PZT is appliedto piezoelectric elements, a trigonal crystal region with a Zr-richcomposition is used. In this case, Zr-rich PZT is called soft PZT. Thisliterally means that the crystal is soft. Soft PZT is used in a nozzlethat ejects ink in an inkjet printer, but since it is excessively soft,ink that is too viscous would impart stress thereto, making itimpossible to push out.

Ti-rich tetragonal PZT, on the other hand, is called hard PZT, whichmeans it is hard and brittle. While the PZTN film of the presentinvention is hard, the crystals can be changed into trigonal crystals byartificial means. Since it is also possible to change the crystal formarbitrarily by the quantity of added Nb and since a Ti-rich PZT-familyferroelectric film has a small relative permittivity, it is possible todrive such a component at a low voltage.

This makes it possible to use hard PZT in applications in which it couldnot be used previously, such as in the ink ejection nozzles of an inkjetprinter. In addition, since Nb makes the PZT softer, it is possible toprovide a PZT that is suitably hard, but not brittle.

Finally, it is also possible to reduce the crystallization temperatureof this embodiment by adding not just Nb, but a silicate simultaneouslywith the addition of the Nb.

THIRD EMBODIMENT

This embodiment investigates the validity of using a PZTN film from theviewpoint of lattice regularity, when the PZTN film has been formed on ametal film formed of a platinum-group metal such as Pt or Ir as anelectrode material for a ferroelectric capacitor that forms a memorycell portion of ferroelectric memory or a piezoelectric actuator thatconfigures an ink ejection nozzle portion of an inkjet printer, by wayof example. Platinum-group metals act as underlayer films that determinethe crystal orientation of ferroelectric films, and are also useful aselectrode materials. However, since the lattice regularities of the twomaterials are not sufficient, a problem arises concerning the fatiguecharacteristics of ferroelectric films when applied to elements.

In this case, the present inventors have developed a technique designedto ameliorate lattice mismatches between a PZT-family ferroelectric filmand a platinum-group metal thin film, by incorporating Nb into theconstituent elements of the PZT-family ferroelectric film. The processof manufacturing this PZT-family ferroelectric film is shown in FIGS.23A to 23C.

First of all, a given substrate 11 was prepared, as shown in FIG. 23A. ATiOx layer formed on an SOI substrate was used as the substrate 11. Notethat a preferred material could be selected from known materials as thissubstrate 11.

Next, as shown in FIG. 23B, a metal film (first electrode) 102 is formedby sputtering Pt, by way of example, onto the substrate 11, then a PZTNfilm is formed as the ferroelectric film 101 on the metal film 102, asshown in FIG. 23C. sol-gel solutions can be used as the materials forforming the PZTN film. More specifically, a mixture of a sol-gelsolution for PbZrO₃, a sol-gel solution for PbTiO₃, and a sol-gelsolution for PbNbO₃ is used, with a sol-gel solution for PbNbO₃ addedthereto. Note that since a constituent element of the PZTN film is Nb,the crystallization temperature thereof is high. For that reason, it ispreferable to further add the sol-gel solution for PbSiO₃, to reduce thecrystallization temperature. With this embodiment, the abovementionedsol-gel mixed solution is painted onto the Pt metal film 102 by aspin-coating method, then is subjected to predetermined thermalprocessing to crystallize it. The film formation flow was similar toshown in FIG. 2.

When an X-ray diffraction method was used to measure the crystal latticeconstants of PZTN films obtained by this embodiment of the invention,wherein the quantity of added Nb ranged from 0 mol % to 30 mol %, theresults were as shown in FIGS. 24A and 24B. It is clear from FIGS. 24Aand 24B that the lattice constant along the a axis (or the b axis)became closer to the lattice constant along the c axis as the quantityof added Nb increased. In addition, V(abc) in FIG. 24A is an index ofvolumetric changes in lattice constant (a,b,c). The ratio V/V₀ in FIG.24A is the ratio of the volume V(abc) of the PZTN crystal with respectto an index V₀ which is a volumetric change of the lattice constant of aPZT crystal to which no Nb was added. It can be confirmed from theV(abc) or V/V₀ column that the crystal lattice of the PZTN crystalbecomes smaller as the quantity of added Nb increases.

The lattice mismatch ratios with respect to a lattice constant(a,b,c=3.96) for Pt metal film were calculated from the latticeconstants of PZTN films formed with the addition of Nb in this manner,and the quantity of added Nb (mol %) was plotted along the horizontalaxis in FIG. 25. It was confirmed from FIG. 25 that the effects ofincluding Nb into a PZT-family ferroelectric film are not limited to theeffect of improving the ferroelectric characteristic of each of theabove described embodiments, but they also include the effect ofapproximating the lattice constant thereof to the lattice constant ofcrystals of platinum-group metals such as Pt. It was confirmed that thiseffect is particularly obvious in the region in which the quantity ofadded Nb is greater than or equal to 5 mol %.

It has therefore been confirmed that use of the method of the presentinvention reduces lattice mismatches between the metal film that is theelectrode material and the ferroelectric film, such that the latticemismatch ratio is improved to the order of 2% at a quantity of added Nbof 30 mol %, by way of example. This is considered to be because strongbonds having both ionic bonding between Nb atoms that have substitutedfor Ti atoms at the B sites in the PZTN crystal structure and O atomsand covalence, these bonds act in directions that compress the crystallattice, causing changes in the direction in which the lattice constantdecreases.

In addition, platinum-group metals such as Pt are chemically stablesubstances that are ideal as the electrode material for ferroelectricmemory or a piezoelectric actuator, so that the method of thisembodiment makes it possible to alleviate lattice mismatches more thanin the conventional art, even when a PZTN film is formed directly onthis Pt metal film, and also improve the interface characteristicthereof. The method of this embodiment therefore makes it possible toreduce fatigue deterioration of PZT-family ferroelectric films, makingit suitable for application to elements such as ferroelectric memory orpiezoelectric actuators.

REFERENCE EXAMPLE

For this example, PbZr_(0.4)Ti_(0.6)O₃ ferroelectric films weremanufactured.

A solution including approximately 20% excess Pb is used for theconventional method, but this is to restrain volatile Pb and reduce thecrystallization temperature. However, since it is unclear what happensto excess Pb in the completed thin films, excessive quantities of Pbshould be suppressed to a minimum.

In this case, a 10 wt % density of a sol-gel solution forPbZr_(0.4)Ti_(0.6)O₃ (solvent: n-butanol) having 0, 5, 10, 15, or 20%excess Pb was used, to which was added 1 mol % of 10 wt % density of asol-gel solution for forming PbSiO₃ (solvent: n-butanol), was used toform 200-nm PbZr_(0.4)Ti_(0.6)O₃ ferroelectric films by the processes ofsteps ST20 to ST25 of FIG. 26. The surface morphologies in this case areshown in FIGS. 27A to 27C and XRD patterns thereof are shown in FIGS.28A to 28C.

Although an excess of approximately 20% Pb is necessary in theconventional art, it is clear that crystallization proceeds sufficientlywith a 5% excess of Pb. This shows that the addition of just 1 mol % ofthe PbSiO₃ catalyst lowers the crystallization temperature of PZT, sothat most of the excess Pb is not needed. Thereonafter the solutions forforming PZT, PbTiO₃, and PbZrTiO₃ all had 5% excess Pb.

Next, a mixed solution of 10 wt % density of a sol-gel solution forforming PbZrO₃ (solvent: n-butanol) and 10 wt % density of a sol-gelsolution for forming PbTiO₃ (solvent: n-butanol) in the ratio 4:6, towhich was added 1 mol % of 10 wt % density of a sol-gel solution forforming PbSiO₃ (solvent: n-butanol), was used in accordance with theflow shown in FIG. 2 to manufacture 200-nm PbZr_(0.4)Ti_(0.6)O₃ferroelectric films. The hysteresis characteristics in this case werefavorable, as shown in FIGS. 29A and 29B. However, it was clear theywere simultaneously leaky.

For comparison, a 200-nm PbZr_(0.4)Ti_(0.6)O₃ ferroelectric thin filmwas manufactured by a conventional method and the previously-describedflow of FIG. 26, using a mixed solution of 10 wt % density of a sol-gelsolution for forming PbSiO₃ (solvent: n-butanol) in 10 wt % density of asol-gel solution for PbZr_(0.4)Ti_(0.6)O₃ (solvent: n-butanol). Thehysteresis characteristic in this case was not particularly impressive,as shown in FIG. 30.

When degassing analysis was performed on each of these ferroelectricfilms, the results were as shown in FIGS. 31A and 31B.

As shown in FIG. 31A, it was confirmed that the conventionalferroelectric film manufactured by PZT sol-gel solutions always degaseswith respect to H and C, as the temperature rises from room temperatureto 1000° C.

With the ferroelectric film of the present invention formed by using asolution that is a 4:6 mixture of 10 wt % density of a sol-gel solutionfor forming PbZrO₃ (solvent: n-butanol) and 10 wt % density of a sol-gelsolution for forming PbTiO₃ (solvent: n-butanol), however, analysisshowed that degassing was mostly not seen.

This is thought to be because the use of a solution that is a 4:6mixture of 10 wt % density of a sol-gel solution for forming PbZrO₃(solvent: n-butanol) and 10 wt % density of a sol-gel solution forforming PbTiO₃ (solvent: n-butanol) ensures that PbTiO₃ crystallizes onthe Pt from the 10 wt % density of the sol-gel solution for formingPbTiO₃ (solvent: n-butanol) within the initial mixed solution and thisacts as initial crystallization seeds, and also that lattice mismatchesbetween the Pt and the PZT disappear, facilitating the crystallizationof the PZT. The use of a mixed solution is also considered to form asuitable interface between the PbTiO₃ and the PZT, which is linked tofavorable squareness of hysteresis loop.

2. Method of Manufacturing Ferroelectric Capacitor

Sections showing an example of a method of manufacturing a ferroelectriccapacitor in accordance with this embodiment of the invention are shownschematically in FIGS. 32A to 32C.

1) First of all, as shown in FIG. 32A, a lower electrode 102, theferroelectric film 101, and an upper electrode 103 are formed insequence as a stack on a given substrate 110.

The substrate 110 is not particularly limited and thus any preferredsubstance can be used therefore, depending on the application of theferroelectric capacitor, such as a semiconductor substrate or a resinsubstrate, by way of example.

Either a precious metal such as Pt, Ir, or Ru alone or a compoundmaterial having such a precious metal as a main component can beemployed as the lower electrode 102 and the upper electrode 103. A knownfilm formation method could be used for forming the lower electrode 102and the upper electrode 103, such as sputtering or vapor deposition.Since the diffusion of ferroelectric elements into the lower electrode102 or the upper electrode 103 would cause variations in the compositionof the interface between that electrode and the ferroelectric film 101,which would adversely affect the squareness of the hysteresis loop, acompact structure that does not permit the diffusion of ferroelectricelements into the lower electrode 102 or the upper electrode 103 isdesired. In this case, a method of forming the films by sputtering by agas having a large mass, or a method of dispersing an oxide of asubstance such as Y or L into a precious metal electrode could beemployed in order to increase the compactness of the lower electrode 102and the upper electrode 103.

The ferroelectric film 101 includes Pb, Zr, Ti, and Nb as constituentelements, and thus is called a PZTN complex oxide. The ferroelectricfilm 101 can be formed by using a spin-coating method or the like topaint sol-gel solutions including Pb, Zr, Ti, and Nb onto the lowerelectrode 102. Mixtures of a first sol-gel solution in which acondensation polymer for forming PbZrO₃ Perovskite crystals by Pb and Zris dissolved in a non-aqueous state in a solvent such as n-butanol; asecond solution in which a condensation polymer for forming PbTiO₃Perovskite crystals by Pb and Ti, from among constituent metal elementsfor the PZTN ferroelectric phase, is dissolved in a non-aqueous state ina solvent such as n-butanol; and a third sol-gel solution in which acondensation polymer for forming PbTiO₃ Perovskite crystals by Pb andTi, from among constituent metal elements for the PZTN ferroelectricphase, is dissolved in a non-aqueous state in a solvent such asn-butanol could be used as these sol-gel solutions. In addition, duringthe formation of the ferroelectric film 101, a sol-gel solutionincluding a silicate or germanate for reducing the crystallizationtemperature of the PZTN complex oxide could be added. More specifically,at least 1 mol % but less than 5 mol % of a fourth sol-gel solution inwhich a condensation polymer for forming PbSiO₃ crystals is dissolved ina non-aqueous state in a solvent such as n-butanol could be furtheradded to the above-described mixture of sol-gel solutions. The mixing inof this fourth sol-gel solution makes it possible for thecrystallization to occur within a temperature range that enables thecreation of elements at a crystallization temperature for the PZTNcomplex oxide of 700° C., although the inclusion of Nb as a constituentelement would normally increase the crystallization temperature.

It is preferable that the painted film for the ferroelectric film 101 issubjected to preliminary thermal processing at a temperature (such asnot more than 400° C. ) that does not cause crystallization of the PZTNcomplex oxide in an oxidizing atmosphere, to put the PZTN complex oxideinto an amorphous state. This enables the advance of the previouslydescribed process while preventing the diffusion of constituent elementsin a state in which the ferroelectric film 101 is in an amorphous state,with no grain boundaries. The performing of this preliminary thermalprocessing in an oxidizing atmosphere has the effect of introducing intothe ferroelectric film 101 the oxygen component that is necessary forthe crystallization of the PZTN complex oxide after the formation of aprotective film, which will be described layer.

2) Next, as shown in FIG. 32B, the lower electrode 102, theferroelectric film 101, and the upper electrode 103 are etched to adesired shape, and a protective film 104 of silicon dioxide (SiO₂) isformed to cover them. The protective film 164 in this case can be formedby a CVD method, using trimethylsilane (TMS). With trimethylsilane(TMS), there is a smaller quantity of hydrogen generated during the CVDprocess, in comparison with the tetraethyl orthosilicate (TEOS) that isgenerally used for forming a silicon dioxide film. If trimethylsilane(TMS) is used for that reason, it is possible to reduce processingdamage to the ferroelectric film 101 due to the reductive reaction.Since the process of using trimethylsilane (TMS) to form the protectivefilm 104 can be done at a lower temperature (from room temperature to350° C.) than the process using TEOS (a film-formation temperature of atleast 400° C.), it is possible to maintain the amorphous state achievedby the process of (1), preventing crystallization of the PZTN complexoxide by the heat generated by this process of forming the protectivefilm 104.

3) Next, as shown in FIG. 32C, thermal processing is performed tocrystallize the PZTN complex oxide that configures the ferroelectricfilm 101, making it possible to obtain a ferroelectric capacitor havinga PZTN ferroelectric crystal film 101 a. This thermal processing couldbe done, not in an oxygen atmosphere, but in an atmosphere of anon-oxidizing gas such as Ar or N₂ or in air, to enable thecrystallization of the PZTN complex oxide.

FIGS. 33A and 33B show results obtained by measuring the hysteresischaracteristics of capacitors in which the manufacture method of thisembodiment was employed to form a SiO₂ protective film by using TMS overa ferroelectric capacitor formed of a Pt lower electrode, a PZTNferroelectric film, and a Pt upper electrode, when the PZTNferroelectric film was subjected to thermal processing in an oxygenatmosphere or air after this SiO₂ protective film was formed. FIG. 33Ashows the results of thermal processing in an oxygen atmosphere and FIG.33B shows the results of thermal processing in air. FIGS. 33A and 33Bshow that hysteresis characteristics with good squareness were obtained,regardless of whether the thermal processing was done in an oxygenatmosphere or air, even though a hydrogen-resisting barrier film was notformed. This is because preliminary thermal processing was performed inan oxidizing atmosphere during the formation of the ferroelectric film101 so that the oxygen necessary for the crystallization had previouslyentered the film. In other words, the manufacture method of thisembodiment makes it possible to crystallize the ferroelectric filmwithout being dependent on the atmosphere for thermal processing. Inaddition, when the thermal processing for crystallization is performedin a non-oxidizing gas atmosphere, it is possible to prevent oxidationdamage due to high-temperature thermal processing on peripheralcomponents (for example, metal wiring) other than the capacitor, whenapplied to a method of manufacturing a ferroelectric memory that will bedescribed later. Note that since the thermal processing forcrystallizing the PZTN complex oxide in this process is not verydependent on the type of gas in the atmosphere, contact holes forforming metal wiring for connecting the upper electrode 103 to theexterior can be formed after the protective film 104 is formed.

FIG. 34 shows the results of measurements obtained by measuring thehysteresis characteristic for examples in which the manufacture methodof this embodiment was employed to form a SiO₂ protective film by usingTMS over a ferroelectric capacitor formed of a Pt lower electrode, aPZTN ferroelectric film, and a Pt upper electrode, and the PZTNferroelectric film was crystallized after the formation of this SiO₂protective film, where the formation temperature of the SiO₂ protectivefilm was room temperature, 125° C., and 200° C.; and the hysteresischaracteristic of a reference example in which the PZTN ferroelectricfilm was crystallized without the SiO₂ protective film being formed, andcalculating the corresponding change in residual polarization magnitude2 Pr. From FIG. 34 it can be seen that there was no change in residualpolarization magnitude 2 Pr, whether the SiO₂ protective film was formedat room temperature, 125° C., or 200° C., which confirms that theformation of the SiO₂ protective film does not result in an inferiorproduct. In other words, by performing the thermal processing forcrystallizing the PZTN complex oxide even after damage is done byhydrogen during the processing of the ferroelectric film 101 in theformation of the protective film 104, the manufacture method of thisembodiment ensures that the PZTN complex oxide is crystallized while anysuch damage is repaired. This makes it possible to omit the process offorming a barrier film for protecting against reductive reactions of theferroelectric film 101, which is necessary in the conventional art,enabling an increase in productivity and a reduction in productioncosts.

3. Ferroelectric Memory

The configuration of a simple matrix type of ferroelectric memory device300 in accordance with an embodiment of the present invention is shownin FIGS. 35A and 35B. FIG. 35A is a plan view thereof and FIG. 35B is asection taken along a line A-A in FIG. 35A. The ferroelectric memorydevice 300 has a predetermined array of word lines 301 to 303 and apredetermined array of bit lines 304 to 306 formed on a substrate 308. Aferroelectric film 307 formed of the PZTN described with respect to thisembodiment is inserted between the word lines 301 to 303 and the bitlines 304 to 306, and ferroelectric capacitors are formed at theintersection regions between the word lines 301 to 303 and the bit lines304 to 306.

In the ferroelectric memory device 300 in which memory cells configuredof this simple matrix are arrayed, the operations of writing to andreading from the ferroelectric capacitors formed at the intersectionsbetween the word lines 301 to 303 and the bit lines 304 to 306 are doneby peripheral drive circuits and a read amplifier circuit (called“peripheral circuit” although not shown in the figures). This peripheralcircuit could be formed of MOS transistors on another substrate than thememory cell array, or the peripheral circuit could be integrated on thesame substrate as the memory cell array.

FIG. 36 is a section through an example of the ferroelectric memorydevice 300 in which the memory cell array is integrated on the samesubstrate as the peripheral circuit.

In FIG. 36, a MOS transistor 402 is formed on a monocrystalline siliconsubstrate 401, and this transistor formation region supports aperipheral circuit. The MOS transistor 402 is formed of themonocrystalline silicon substrate 401, a source/drain region 405, a gateisolation film 403, and a gate electrode 404.

The ferroelectric memory device 300 includes an element separation oxidelayer 406, a first interlayer dielectric 407, a first wiring layer 408,and a second interlayer dielectric 409.

The ferroelectric memory device 300 has a memory cell array formed offerroelectric capacitors 420, where each ferroelectric memory cell isformed of a lower electrode (first electrode or second electrode) 410that becomes a word line or bit line, a ferroelectric film 411 includinga ferroelectric phase and an ordinary paraelectric phase, and an upperelectrode (second electrode or first electrode) 412 that becomes a bitline or a word line.

This ferroelectric memory device 300 also has a third interlayerdielectric 413 on the ferroelectric capacitor 420, and the memory cellarray and the peripheral circuit are connected by a second wiring layer414. Note that a protective film 415 is formed over the third interlayerdielectric 413 and the second wiring layer 414 of the ferroelectricmemory device 300.

The ferroelectric memory device 300 having the configuration describedabove makes it possible to integrate a memory cell array and aperipheral circuit on the same substrate. Note that the ferroelectricmemory device 300 shown in FIG. 36 is configured of a memory cell arrayon top of the peripheral circuit, but the configuration could equallywell be such that the memory cell array is connected to the peripheralcircuit in a planar manner, without disposing the memory cell array onthe peripheral circuit.

Since the ferroelectric capacitor 420 used in this embodiment isconfigured of the PZTN described above, the squareness of the hysteresisloop is extremely good and it has a stable disturbance characteristic.In addition, the reduction in the processing temperature for thisferroelectric capacitor 420 reduces damage to peripheral circuits andother components, and also reduces processing damage (particularly thatdue to hydrogen reduction), so that any deterioration in the hysteresisloop due to damage can be suppressed. The use of this ferroelectriccapacitor 420 therefore enables practicable application of the simplematrix type of ferroelectric memory device 300.

A configurational view of a 1T1C type of ferroelectric memory device 500that is a variant example is shown in FIG. 37A. an equivalent circuitdiagram of the ferroelectric memory device 500 is shown in FIG. 37B.

The ferroelectric memory device 500 is a memory device of aconfiguration that closely resembles DRAM, having a capacitor 504 (1C)formed of a lower electrode 501, an upper electrode 502 connected to aplate line, and a ferroelectric film 503 to which the PZTN ferroelectricof this embodiment is applied; and a transistor element 507 (1T) forswitching wherein either the source or the drain electrode is connectedto a data line 505 and a gate electrode 506 is connected to a word line,as shown in FIG. 37A. There are hopes that this structure will replaceSRAM since writing and reading with respect to a 1T1C type of memory canbe done at high speeds of not more than 100 ns and also the data writtenthereto is non-volatile.

4. Method of Manufacturing Ferroelectric Memory

The description now turns to a case in which the manufacture methoddescribed in “2. Method of Manufacturing Ferroelectric Capacitor” isapplied to a method of manufacturing ferroelectric memory.

FIGS. 38A to 38C are schematic sections showing an example of theprocess of manufacturing the ferroelectric memory in accordance withthis embodiment.

With this embodiment, the lower electrode 102, the PZTN ferroelectricfilm 101, and the upper electrode 103 of the ferroelectric capacitor 100are formed sequentially on the substrate 110, as shown in FIG. 38A.During this time, the ferroelectric film 101 is subjected to preliminarythermal processing in an oxidizing atmosphere, to put it into anamorphous state. Note that the substrate 110 could have a configurationsuch that a transistor 116 for cell selection is formed on asemiconductor substrate 111, as shown by way of example in FIG. 38A.This transistor 116 could be configured of a source and drain 113, agate oxide layer 114, and a gate electrode 115. A plug electrode 117formed of tungsten or the like is formed over either the source or thedrain 113, enabling the use of a stack structure that is formed toenable connection to the lower electrode 102 of the ferroelectriccapacitor 100. The cells are isolated by an element separation region112 in the substrate 110 between the cells, and the transistor 116 canhave an interlayer dielectric 118 formed of an oxide layer or the likeabove the transistor 116.

The fabrication process in accordance with this embodiment patterns theferroelectric capacitor 100 to the desired size and shape, as shown inFIG. 38B. The SiO₂ protective film 104 is formed by usingtrimethylsilane (TMS) to cover the ferroelectric capacitor 100, acontact hole 105 for connection to the exterior is formed, and thenthermal processing is preformed to crystallize the PZTN ferroelectricand form the ferroelectric film 101 a. During the crystallization of thePZTN ferroelectric, the thermal processing for the crystallization couldbe performed in a non-oxidizing atmosphere, This makes it possible toprevent any oxidation damage due to high-temperature thermal processingto peripheral components (such as metal wiring) outside of theferroelectric capacitor 100.

Finally, a contact hole for connecting the transistor 116 to theexterior is formed in the SiO₂ protective film 104 and the ferroelectricmemory is completed by the formation of metal wiring layers 191 and 192,as shown in FIG. 38C. The fabrication process of this embodiment enablesthe omission of the process of forming a barrier film for protecting theferroelectric film 101 from reductive reactions, which is necessary inthe conventional art, thus enabling an increase in productivity and areduction in production costs. Since this enables the formation of theferroelectric capacitor 100 that has a favorably square hysteresischaracteristic even although the process of forming that barrier layeris omitted, it makes it possible to obtain ferroelectric memory withsuperlative characteristics.

Note that the descriptions above dealt with the process of manufacturinga 1T1C type of ferroelectric memory but the method of manufacturing aferroelectric capacitor in accordance with this embodiment can also beapplied to methods of manufacturing ferroelectric memory that use othertypes of cell, such as the 2T2C type or the simple matrix type(crosspoint-type).

5. Piezoelectric Element and Inkjet-Type Recording Head

The description now turns to details of an inkjet type of recording headin accordance with an embodiment of the present invention.

In an inkjet-type recording head wherein part of a stress generatingchamber that communicates with a nozzle aperture that ejects inkdroplets is formed of a vibrating plate, where ink in the stressgenerating chamber is pressurized by distortions of this vibrating plateby a piezoelectric element and is ejected as ink droplets from a nozzleaperture, there are two methods of implementation: one using apiezoelectric actuator having longitudinal resonance mode by which apiezoelectric element resonates longitudinally to expand and contract inthe axial direction and one using an piezoelectric actuator having aflexural resonance mode.

It is known to form a uniform piezoelectric layer by a film-formationtechnique over the entire surface of a vibrating plate, for use as anactuator of a flexural resonance mode, then divide that piezoelectriclayer by a lithography method into shapes corresponding to stressgenerating chambers and form a piezoelectric element independently foreach stress generating chamber.

A partial perspective view of parts of an inkjet-type recording head inaccordance with an embodiment of the present invention is shown in FIG.39, a plan view and a section taken along the line A-A′ of FIG. 39 areshown in FIGS. 40A and FIG. 40B, and a schematic view of the layerstructure of a piezoelectric element 700 is shown in FIG. 41. As shownin these figures, a flow path shaping substrate 10 is formed of a(110)-orientation silicon monocrystalline substrate in accordance withthis embodiment, and an elastic film 50 of thickness 1 to 2 μm is formedof silicon dioxide by previous thermal oxidation on one surface thereof.A plurality of stress generating chambers 12 are arrayed in thewidthwise direction of the flow path shaping substrate 10. A connectiveportion 13 is formed in the longitudinal direction of a region on theouter side of the stress generating chambers 12 of the flow path shapingsubstrate 10 and the connective portion 13 and the stress generatingchambers 12 communicate through an ink supply path 14 provided for eachstress generating chamber 12. Note that the connective portion 13 formspart of a reservoir 800 that forms a common ink chamber for the stressgenerating chambers 12 communicating with a reservoir portion of asealing substrate 30 that will be described later. Each ink supply path14 is formed to width that is narrower than the stress generatingchamber 12, to keep the resistance of ink flowing into the stressgenerating chamber 12 from the ink supply path 14 constant.

On an aperture surface side of the flow path shaping substrate 10, anozzle plate 20 is affixed by adhesive or thermal bonding film. Nozzleapertures 21 pierce the nozzle plate 20 and communicate with an edgeportion on the opposite side from the ink supply paths 14 of the stressgenerating chambers 12.

On the opposite side from the aperture surface of the flow path shapingsubstrate 10, the elastic film 50 of a thickness of approximately 1.0μm, by way of example, is formed as mentioned previously, and adielectric film 55 of a thickness of approximately 0.4 μm, by way ofexample, is formed on that elastic film 50. In addition, a lowerelectrode film 60 of a thickness such as approximately 0.2 μm, apiezoelectric layer 70 of a thickness such as approximately 1.0 μm, andan upper electrode film 80 of a thickness such as approximately 0.05 μmare formed in a stack on the dielectric film 55 by processing that willbe described later, to form the piezoelectric element 700. In this case,the piezoelectric element 700 is the portion including the lowerelectrode film 60, the piezoelectric layer 70, and the upper electrodefilm 80. In general, one electrode of the piezoelectric element 700 is acommon electrode and the other electrode and the piezoelectric layer 70are patterned to form each stress generating chamber 12. A portionformed of the thus-patterned electrode and the piezoelectric layer 70that generates piezoelectric strain by the application of a voltage tothe two electrodes is called an active piezoelectric portion. With thisembodiment, the lower electrode film 60 is the common electrode of thepiezoelectric element 700 and the upper electrode film 80 is the otherelectrode of the piezoelectric element 700, but there is no obstacle toreversing these roles to suit the circumstances of the drive circuit orwiring. In either case, an active piezoelectric portion is formed foreach stress generating chamber. In this case, the combination of thepiezoelectric element 700 and the vibrating plate in which displacementsare generated by the driving of that piezoelectric element 700 is calleda piezoelectric actuator. Note that the piezoelectric layer 70 isprovided independently for each stress generating chamber 12 and isconfigured of a plurality of layers of ferroelectric film 71 (71 a to 71f).

An inkjet-type recording head forms part of a recording head unit thatprovides an ink flow path communicating with an ink cartridge or thelike, and is mounted in an inkjet-type recording device. A schematicview of an example of this inkjet-type recording device is shown in FIG.42. As shown in FIG. 42, recording head units 1A and 1B havinginkjet-type recording heads are provided with removable cartridges 2Aand 2B that form ink supply means, and a carriage in which theserecording head units 1A and 1B are mounted is provided so as to be ableto move freely in the axial direction of a carriage shaft 5 that isattached in a device body 4. These recording head units 1A and 1B aredesigned to eject substances that compose black ink and color ink,respectively. The carriage 3 in which the recording head units 1A and 1Bare mounted is made to move along the carriage shaft 5 by the transferof a driving force of a drive motor 6 to the carriage 3 through aplurality of gearwheels (not shown in the figure) and a timing belt 7. Aplaten 8 is also provided on the carriage shaft 5 in the device body anda sheet S that is a sheet of a recording medium such as paper suppliedby paper-supply rollers (not shown in the figure) is transferred ontothe platen 8.

Note that the description above relates to one example of an inkjet-typerecording head that ejects ink as a liquid ejection head, but thepresent invention can also be applied widely to liquid ejection headsand liquid ejection devices in which piezoelectric elements are used.Examples of such a liquid ejection head include a recording head used inan image recording device such as a printer, a color jet head used forforming a color filter for a liquid-crystal display or the like, anorganic EL display, an electrode material ejection head used for formingelectrodes for a field emission display (FED) or the like, and a livingorganic ejection head used in the formation of biochips.

Since the piezoelectric element of this embodiment uses a PZTN film inaccordance with this embodiment as described above as the piezoelectriclayer, it achieves the following effects.

1) Since covalence is increased in the piezoelectric layer, thepiezoelectric constant is increased.

2) It is easy to apply an electrical field that suppresses thegeneration of faults in the interface between the piezoelectric layerand the electrode, for suppressing any insufficiency of PbO in thepiezoelectric layer, enabling an increase in the efficiency thereof as apiezoelectric element.

3) Since leakage currents in the piezoelectric layer are suppressed, itis possible to make a thin film of the piezoelectric layer.

A liquid ejection head and liquid ejection device in accordance withthis embodiment makes use of a piezoelectric element including theabove-described piezoelectric layer, enabling it to achieve thefollowing effect in particular.

4) Since a reduction in fatigue deterioration of the piezoelectric layeris enabled, time-related changes in the displacement magnitude of thepiezoelectric layer can be suppressed, enabling an increase inreliability.

The present invention has been described above with reference topreferred embodiments thereof but the present invention is not limitedthereto and thus is it possible to implement other types of distortionwithin the range of the invention as laid out herein.

For example, the substitution of Ta, W, V, or Mo for the Nb in the PZTof the ferroelectric film 101, or the addition thereof, would havesimilar effects. The use of Mn as an addition would have effects similarto those of Nb. Similar thinking would lead to the substitution ofelements of a valence of +3 or greater, to prevent the escape of Pb, andcandidates therefor could be of the lanthanoide series, such as La, Ce,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. In addition theadditive that promotes crystallization could be a germanate (Ge) insteadof a silicate. The hysteresis characteristic of a material in which 10mol % Ta is added to the PZT in place of Nb is shown in FIG. 43A. Thehysteresis characteristic of a material in which 10 mol % W is added tothe PZT in place of Nb is shown in FIG. 43B. It is clear that the use ofTa would have a similar effect to that obtained by the addition of Nb.Similarly, the use of W have also a similar effect to that obtained bythe addition of Nb, from the viewpoint of a hysteresis characteristichas good insulating properties.

1. A ferroelectric film that is described by AB_(1-X)Nb_(X)O₃, whereinan A element includes at least Pb, wherein a B element includes at leastZr and Ti, wherein X is within the range of: 0.05≦X≦0.4, and furthercomprising Si.
 2. The ferroelectric film as defined by claim 1, whereinSi is within the range of: 0.5 to 5 mol %.
 3. The ferroelectric film asdefined by claim 1, further comprising Ge.
 4. The ferroelectric film asdefined by claim 1, having a crystal structure of at least one oftetragonal and rhombohedral systems.
 5. The ferroelectric film asdefined by claim 1, wherein an amount of Pb vacancy is less than 20 mol% of a stoichiometric composition of the AB_(1-X)Nb_(X)O₃.
 6. Theferroelectric film as defined by claim 5, wherein the amount of Pbvacancy is half of X.
 7. A ferroelectric memory device comprising: asubstrate, a transistor formed on the substrate, a ferroelectriccapacitor formed above the substrate, and wherein the ferroelectriccapacitor comprises a ferroelectric film as defined by claim
 1. 8. Apiezoelectric actuator comprising: a substrate, a piezoelectric elementformed above the substrate, and wherein the piezoelectric elementcomprises a ferroelectric film as defined by claim
 1. 9. A ferroelectricfilm that is described by AB_(1-X)Nb_(X)O₃, wherein an A elementincludes at least Pb, wherein a B element includes at least Zr and Ti,and wherein X is within the range of: 0.1<X≦0.4.